1. Field of the Invention
This invention relates to a liquid crystal projection display, and particularly to a field-sequential dichroic light valve.
2. Prior Art of the Invention
A conventional electronic optical switch can be turned ON/OFF by the rotation of light polarization by a twisted neumatic (TN) liquid crystal component and the effect of an external electric field to the direction of LC principal axis. Referring to FIGS. 1a and 1b, the unpolarized incident light Io becomes a polarized light having an a-direction polarization after passing through the polarization film 10. The a-direction polarized light changes the polarization direction to b-direction with the effect of the alignment films 12a, 12b and the liquid crystal molecules 14 in the cavity. Therefore, the polarized light can be emitted from the polarized film 16. In FIG. 1b, the liquid crystal molecules 14 will not change the polarization direction of incident light due to the effect of the external electric field of voltage source 18. Therefore, the a-direction-polarized light cannot be emitted through the polarized film 16. The conventional optical switch is not operated in a mechanical manner and has a high contrast. The switch speed of the optical switch can achieve the level of 20xcx9c50 msec. However, such a switching speed is not fast enough for a field sequential liquid crystal display, especially for a tri-color projection display system (PDS).
Other prior arts, such as U.S. Pat. No. 5,868,482 of Edlinger et al., U.S. Pat. No. 5,448,314 of Heimbuch et al., and U.S. Pat. No. 5,371,543 of Anderson, utilize the color wheel technology used in conventional TV and PDS products. Operated in a mechanical manner, these prior arts have a low S/N ratio, and it is hard to adjust the time sequence. Furthermore, the different color sectors on the wheel waste optical energy.
Recently, ferro-electric liquid crystal cell (FLC-CELL) products, such as the FLC driving scheme of Displaytech Co., have raised the switch speed to 20 xcexcsec, and have memory functions that TN LC cells do not have. However, like a TN LCD panel, a FLCD panel needs a dyed PVA polarizer, which has a high absorption for visible light. The light transmittance of a FLCD panel is only about 28xcx9c30%. Energy loss is a serious problem for TN LCD and FLCD panels.
To improve the light transmittance of a polarizer, a structure of all dielectric multi-layer film is of interest. However, it is very difficult to design the film system. In the optical communications field, combining a multi-layer film polarized beam splitter with an electric switch to form a single unit has been reported. Many single units are serially arranged to control the direction of the optical path for a predetermined wavelength. However, this device does not have light-combining or light-splitting function. The structure of this device is as shown in FIG. 2., wherein the reference numbers 21xcx9c26 indicate optical fibers, reference numbers 27xcx9c32 indicate cylinder lenses, reference numbers 33xcx9c37 indicate polarized beam splitter (PBS) prisms, reference numbers 38xcx9c41 indicate electro-optical components, and reference numbers 43xcx9c47 indicate PBSs.
To provide an improved LCD system having a fast time sequential switch mechanism, an electrically controlled switch having a high speed and a high transmittance is required.
Accordingly, an object of this invention is to provide a field sequential dichroic light valve, which has a fast time response and a memory function.
Another object of this invention is to provide a field sequential dichroic light valve, which has a switch frequency larger than 180 Hz and an optical transmittance larger than 90%.
With an appropriately designed optical thin-film filter, this invention can provide a field sequential light-splitting/combining module in an optical system having a white light source.
To achieve the above objects, this invention provides a time sequential dichroic light valve, which comprises: a first polarizing beam-splitting device for receiving a broadband incident light and emitting a second polarized light of a first spectral range when an energy field is applied thereto; a second polarizing beam-splitting device for receiving a first polarized light and a second polarized light of other spectral ranges except the first spectral range, and emitting the second polarized light of a second spectral range when an energy field is applied thereto; and a third polarizing beam-splitting device for receiving first polarized light and a second polarized light of other spectral ranges except the first spectral range and the second spectral range, and emitting the second polarized light of a third spectral range when an energy field is applied thereto, whereby the energy field is sequentially applied to the first polarizing beam-splitting device, the second polarizing beam-splitting device and the third polarizing beam-splitting device to switch the transmission of the second polarized lights of the first spectral range, the second spectral range and the third spectral range along a selective direction.